1. Field of the Invention
This invention relates to an improved planetary inker for an offset lithographic printing press wherein an ink form roller and an ink/water form roller provide improved printing quality as well as versatility of plate cylinder size.
2. Description of the Prior Art
Clear, high-quality printing can be obtained from an offset lithographic press only when two substances are supplied to the printing plate in a proper, proportional relationship. The first substance is the dampening fluid or so-called "water", which is actually a solution of water, gum arabic, and a number of other chemicals which have an attraction to the hydrophilic, or non-image areas of the printing plate. The second essential substance in the printing process is the ink, which is an oil based solution comprised of carrying vehicles, pigments, dryers and a range of other miscellaneous ingredients that when combined form a solution having a strong attraction for the oleophilic, or ink receptive image areas of the printing plate.
Typically, severe demands are placed upon the inking system of an offset press. First, the viscous ink must be milled from a thick, plastic state into a thin film around all of the form rollers. Secondly, the ink must then be transferred from the form rollers to a uniform, even film of ink on the image areas of the plate. Additionally, the inking system should pick up particles of lint and other foreign matter from the plate cylinder so that the final printed image on the paper is free of defects.
The inking system should also be operable to fully replenish ink to all image areas of the plate during each revolution of the latter; otherwise, "ghosting" will occur. Ghosting is often encountered on printing which requires large amounts of heavy ink coverage in combination with areas requiring little or no ink coverage. Because the form is usually of different diameter than the plate, ink-deficient areas of the form cause a latent, repeat image to appear at a position different than the original location when the form completes a second revolution.
U.S. Pat. No. 4,223,603 to Faddis, et al. discloses a gear driven planetary inker which is operable to minimize ghosting and deliver a uniform film of ink to the plate cylinder. In brief, the planetary inker of this patent has single form roller in rolling contact with the plate cylinder, and also a number of varisized distributor rollers disposed in rolling contact about the perimeter of the form roller. The distributor rollers include a number of non-oscillable rider rollers which provide ink storage and effect milling of the ink. Also, the distributor rollers include a number of longitudinally oscillable vibrator rollers which mill the ink, provide additional ink storage, and distribute the ink from side to side. The planetary inker is superior to other inkers because as the form roller makes one complete revolution, the large number of nips between the form and the various distributor rollers insure that the ink film is fully replenished and that the film is milled to a uniform thickness.
Typically, single form planetary inkers have been associated with conventional dampening systems wherein a ductor roller oscillates between alternating positions of engagement with a water pan roller and a distributor roller which transmits water to the plate cylinder. Unfortunately, the oscillating nature of the ductor roller slows the response of the dampening system such that the press produces additional waste in coming to equilibrium.
Other types of offset presses in the past have used continuous dampening systems wherein the oscillating ductor roller is eliminated. Instead, the dampening solution is applied to a water dedicated form roller which is in continuous, rolling contact with the plate cylinder. It is an important function of the continuous as well as the conventional dampening system to apply water to the nonprinting areas of the plate before the latter contacts the ink form roller to insure that the plate resists ink deposition in the non-image areas.